1. Field of the Invention
The present invention relates to an image forming apparatus for use in electrophotography. In addition, the present invention also relates to a toner and a process cartridge used for the image forming apparatus.
2. Discussion of the Background
In the field of electrophotography, demands for high-speed printing and high-quality images have increased recently. In particular, the fixability of a toner, which is one of the important factors having an influence on the resultant image quality, deteriorates as the image forming speed (i.e., the system speed of an image forming apparatus) increases. Therefore, it is difficult to realize both good fixability and high-speed printing at the same time.
An unfixed toner image is firmly fixed on a paper by application of heat and pressure in a fixing device. When the system speed increases, the unfixed toner image on the paper may not receive a satisfactory amount of heat. As a result, the toner image may be weakly fixed and tends to peel off from the paper.
In order not to deteriorate fixability even if the system speed increases, the fixing temperature may be increased. However, when the fixing temperature increases, the following problems may arise:    (1) the inner temperature of an image forming apparatus increases and side effects are caused;    (2) the life of a fixing member deteriorates; and    (3) the energy conservation is not achieved.
For the above reasons, the toner itself is required to improve fixability especially for use in an ultrahigh-speed machine. A toner having good fixability even when the image forming speed is high and a low amount of heat is applied is required.
Various attempts have been made to improve fixability of a toner.
For example, a technique of controlling thermal properties of a binder resin is proposed. When the glass transition temperature (Tg) of a binder resin decreases, thermostable preservability and fixing strength of the resultant toner may deteriorate. When the molecular weight of a binder resin decreases, the softening temperature (T(F1/2)) (to be explained in detail later) of the resultant toner decreases. As a result, a hot offset problem, in that part of a fused toner image is adhered and transferred to the surface of a heat member and then the part of the toner image is re-transferred to an undesired portion of a sheet itself or the following sheet of a recording material, is caused and the resultant image has excessively high glossiness. A toner having a good combination of low-temperature fixability, thermostable preservability, and hot offset resistance is not yet obtained only by controlling thermal properties of the binder resin.
In attempting to improve low-temperature fixability of a toner, unexamined published Japanese Patent Applications Nos. (hereinafter referred to as JP-A) 60-90344, 64-15755, 02-82267, 03-229264, 03-41470, and 11-305486 have disclosed toners including a polyester resin potentially having relatively good low-temperature fixability and thermostable preservability, instead of including a styrene-acrylic resin which has been conventionally used.
JP-A 62-63940 discloses a toner including a binder resin including a specific non-polyolefin crystalline polymer capable of sharply melting at the glass transition temperature thereof. However, the molecular structure and the molecular weight of the polymer are not optimized therein.
Japanese Patent No. 2931899 and JP-A 2001-222138 have disclosed techniques in which a binder resin and a toner, respectively, include a crystalline polyester resin capable of sharply melting. However, these techniques are not satisfactory to obtain a toner simultaneously having low-temperature fixability and preservability.
JP-A2002-214833 discloses a toner including a crystalline polyester resin and an amorphous polyester resin, which are incompatible with each other and form a phase separation structure (i.e., a sea-island structure) in the toner. The toner has a specific DSC curve properties. However, this technique is also not satisfactory to obtain a toner simultaneously having low-temperature fixability and preservability.
As mentioned above, the use of a crystalline polyester resin as a binder resin is effective for improving low-temperature fixability of the resultant toner, especially for use in a high-speed or ultrahigh-speed image forming apparatus. However, the crystalline polyester resin tends to cause side effects such as a filming problem in that a toner forms a film thereof on a photoreceptor etc. when an image is formed. If the filming problem seriously occurs, abnormal images such as a solid image having image defects (white spots) tend to be produced. Therefore, a toner satisfying both low-temperature fixability and filming resistance is required so as to be used in an ultrahigh-speed image forming apparatus having a system speed of from 500 to 1700 mm/sec.
JP-A2005-189594 discloses a cleaning device in which toner particles remaining on a photoreceptor are collected by a brush roller. A bias having the same polarity to the toner particles are applied to the brush roller so that the collected toner particles are ejected onto the photoreceptor. When the ejected toner particles pass a cleaning blade, films of the toner formed on the photoreceptor are scraped off. Although the films may be scraped off by the above technique, the fundamental problem of the occurrence of the filming problem cannot be solved.
JP-A2002-278135 discloses a toner including wax particles having a specific circularity. However, this technique is also not satisfactory to obtain a toner simultaneously having good low-temperature fixability and filming resistance. In particular, there is a problem such that the minimum fixable temperature, at which a fixed image has resistance to an abrasion, increases with time.